Radio link monitoring (Radio Link Monitoring, RLM) refers to that user equipment (User Equipment, UE) monitors radio downlink quality of a serving cell on a physical layer, to indicate an in-synchronization (In-Sync, IN) state or an out-of-synchronization (Out-Of-Sync, OOS) state to an upper layer. The UE evaluates downlink quality on the basis of a reference signal and compares the downlink quality with thresholds Qout and Qin. When the evaluated radio link quality is less than the threshold Qout, it is determined that a radio link is in an out-of-synchronization state; or when the evaluated radio link quality is greater than the threshold Qin, it is determined that a radio link is in an in-synchronization state. The threshold Qout is defined as a radio downlink quality level when the UE cannot perform reliable reception on a radio downlink, which is equivalent to that a block error ratio (Block Error Ratio, BLER) of downlink control channel transmission under an assumed condition is 10%. The threshold Qin is defined as a radio downlink quality level when the UE can perform reliable reception on the radio downlink, which is equivalent to that the block error ratio of the downlink control channel transmission under the assumed condition is 2%.
Currently, a new work item (Work Item, WI) is set up in the 3rd Generation Partnership Project (3rd Generation Partnership Project, 3GPP), and machine-type communication (Machine-Type Communication, MTC) UE that performs one-antenna receiving and that has low costs and increasing coverage is introduced. In existing protocols, requirements for radio link monitoring are all for UE performing dual-antenna receiving, and there is no related definition and research for the MTC UE that performs one-antenna receiving and that has low costs and increasing coverage.
In the prior art, a method for performing radio link monitoring by UE performing dual-antenna receiving is shown in FIG. 8. In FIG. 8, the UE needs to monitor downlink quality of a serving cell on a physical layer at an interval of a given time VT, that is, the physical layer of the UE needs to monitor downlink quality of the serving cell at time points T0, T1, T2, . . . , Tn, Tn+1, Tn+2, . . . . Evaluation of radio link quality at a given time point by the UE is obtained on the basis of radio link quality in a time window Tevaluate. For example, the UE needs to evaluate, at a time point Tn, downlink quality in a time segment [T0, Tn] to obtain an evaluation result of link quality corresponding to the time point Tn; and the UE needs to evaluate, at a time point Tn+1, downlink quality in a time segment [T1, Tn+1], to obtain an evaluation result of link quality corresponding to the time point Tn+1.
In the protocol TS36.213, an evaluation time interval (VT in FIG. 8) is defined as:
in a discontinuous reception (Discontinuous Reception, DRX) mode, a physical layer of UE needs to evaluate downlink quality in each DRX period, that is, the evaluation time interval ΔT=one DRX period; and
in a non-DRX mode, the physical layer of the UE needs to evaluate downlink quality in each radio frame, that is, the evaluation time interval ΔT=10 ms.
In the protocol TS36.133, a length of an evaluation time window (Tevaluate shown in FIG. 8) is defined as:
in a DRX mode, a length of an in-synchronization/out-of-synchronization evaluation time window is determined by a length of a DRX period;
in a non-DRX mode, for out-of-synchronization monitoring, the length of the evaluation time window is 200 ms; and
in a non-DRX mode, for in-synchronization monitoring, the length of the evaluation time window is 100 ms.
At beginning, a link connection is normal, and the physical layer of the UE needs to perform out-of-synchronization monitoring. When an evaluation result of downlink quality is less than the threshold Qout, the physical layer of the UE needs to report an out-of-synchronization indication to an upper layer of the UE. After the upper layer of the UE receives N310 consecutive out-of-synchronization indications, the UE needs to enable a timer T310. During enabling of the timer T310, when an evaluation result of downlink quality is greater than the threshold Qin, the physical layer of the UE needs to report an in-synchronization indication to the upper layer of the UE.
If the upper layer of the UE receives N311 consecutive in-synchronization indications before the timer T310 expires (as shown in FIG. 9), it indicates that the downlink recovers normality, and the UE disables the timer T310.
If the upper layer of the UE has not received N311 consecutive in-synchronization indications before the timer T310 expires (as shown in FIG. 10), a radio link failure occurs, and the UE needs to send a radio link failure indication to the serving cell.
A minimum time interval at which two consecutive in-synchronization/out-of-synchronization indication messages are sent in FIG. 9 and FIG. 10 is:
in a DRX mode, the minimum time interval is one DRX period; and
in a non-DRX mode, the minimum time interval is 10 ms.
That is, in the existing protocols, for the UE performing dual-antenna receiving, in the non-DRX mode, the physical layer of the UE needs to evaluate downlink quality in each radio frame, that is, evaluate link quality every 10 ms. For the MTC UE having low costs, one of the most important requirements is “lowest cost (Lowest Cost)”. However, if an evaluation period of 10 ms is directly used for the MTC UE having low costs, excessive power consumption is inevitably caused, which is contrary to a main objective “lowest costs” of the WI. Therefore, it is necessary to provide a new monitoring method to resolve the foregoing problem.